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Diagonal census for the trace-101010 no-four orbit-matrix case

Proved
Conway99.ten_of_no_four_diag_count

by WillR · Sep 8, 2026 · Mathlib 0df444a (Lean v4.33.1)

combinatoricsconway-99orbit-matrixstrongly-regular-graphs

Let CCC be a 9×99\times 99×9 matrix over N\mathbb{N}N whose diagonal entries lie in {0,2,4}\{0,2,4\}{0,2,4}, whose diagonal sums to 101010, and with no diagonal entry equal to 444. Then exactly five diagonal entries equal 222 and four equal 000. Indeed every diagonal entry is 000 or 222, so with c2c_2c2​ entries equal to 222 the trace condition gives 2c2=102c_2 = 102c2​=10 and the nine diagonal positions give c2+c0=9c_2 + c_0 = 9c2​+c0​=9. This pins the row-type multiset used by the profile census in the trace-101010, no-four branch of Wilbrink's Theorem 5.

Preamble
import Mathlib.Data.Matrix.Basic
import Mathlib.Algebra.BigOperators.Fin
Formal statement
open scoped BigOperators

namespace Conway99

theorem ten_of_no_four_diag_count
    (C : Matrix (Fin 9) (Fin 9) ℕ)
    (hdiag : ∀ i, C i i = 0 ∨ C i i = 2 ∨ C i i = 4)
    (htr : ∑ i, C i i = 10)
    (hfour : ∀ i, C i i ≠ 4) :
    (Finset.univ.filter (fun i => C i i = 2)).card = 5 ∧
    (Finset.univ.filter (fun i => C i i = 0)).card = 4 := by sorry

end Conway99
Source
H. A. Wilbrink, 'On the (99,14,1,2) strongly regular graph', EUT Report 84-WSK-03, 1984, Theorem 5, pp. 350-354, https://pure.tue.nl/ws/files/2449333/256699.pdf ; diagonal census for the trace-10, no-four branch.

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